The Ras gene is found activated in many human cancers, including colorectal carcinoma, exocrine pancreatic carcinoma, and myeloid leukemias. Biological and biochemical studies of Ras action indicate that Ras functions like a G-regulatory protein, since Ras must be localized in the plasma membrane and must bind with GTP in order to transform cells (Gibbs, J. et al., Microbiol. Rev. 53: 171-286 (1989). Forms of Ras in cancer cells have mutations that distinguish the protein from Ras in normal cells.
At least 3 post-translational modifications are involved with Ras membrane localization, and all 3 modifications occur at the C-terminus of Ras. The Ras C-terminus contains a sequence motif termed a "CAAX" or "Cys-Aaa.sup.1 -Aaa.sup.2 -Xaa" box (Aaa is an aliphatic amino acid, the Xaa is any amino acid) (Willumsen et al., Nature 310: 583-586 (1984)). Other proteins having this motif include the Ras-related GTP-binding proteins such as Rho, fungal mating factors, the nuclear lamins, and the gamma subunit of transducin.
Farnesylation of Ras by the isoprenoid farnesyl pyrophosphate (FPP) occurs in vivo on Cys to form a thioether linkage (Hancock et al., Cell 57:1167 (1989); Casey et al., Proc. Natl. Acad. Sci. USA 86:8323 (1989)). In addition, Ha-Ras and N-Ras are palmitoylated via formation of a thioester on a Cys residue near a C-terminal Cys farnesyl acceptor (Gutierrez et al., EMBO J. 8:1093-1098 (1989); Hancock et al., Cell 57: 1167-1177 (1989)). Ki-Ras lacks the palmitate acceptor Cys. The last 3 amino acids at the Ras C-terminal end are removed proteolytically, and methyl esterification occurs at the new C-terminus (Hancock et al., ibid). Fungal mating factor and mammalian nuclear lamins undergo identical modification steps (Anderegg et al., J. Biol. Chem. 263: 18236 (1988); Farnsworth et al., J. Biol. Chem. 264: 20422 (1989)).
Inhibition of Ras farnesylation in vivo has been demonstrated with lovastatin (Merck & Co., Rahway, N.J.) and compactin (Hancock et al., ibid; Casey et al., ibid; Schafer et al., Science 245:379 (1989)). These drugs inhibit HMG-CoA reductase, the rate limiting enzyme for the production of polyisoprenoids and the farnesyl pyrophosphate precursor. It has been shown that a farnesyl-protein transferase using farnesyl pyrophosphate as a precursor is responsible for Ras farnesylation. (Reiss et al., Cell, 62: 81-88 (1990); Schaber et al., J. Biol. Chem., 265: 14701-14704 (1990); Schafer et al., Science, 249: 1133-1139 (1990); Manne et al., Proc. Natl. Acad. Sci USA, 87: 7541-7545 (1990)).
Inhibition of farnesyl-protein transferase and, thereby, of farnesylation of the Ras protein, blocks the ability of Ras to transform normal cells to cancer cells. The compounds of the invention inhibit Ras farnesylation and, thereby, generate soluble Ras which, as indicated infra, can act as a dominant negative inhibitor of Ras function. While soluble Ras in cancer cells can become a dominant negative inhibitor, soluble Ras in normal cells would not be an inhibitor.
A cytosol-localized (no Cys-Aaa.sup.1 -Aaa.sup.2 -Xaa box membrane domain present) and activated (impaired GTPase activity, staying bound to GTP) form of Ras acts as a dominant negative Ras inhibitor of membrane-bound Ras function (Gibbs et al., Proc. Natl. Acad. Sci. USA 86: 6630-6634 (1989)). Cytosollocalized forms of Ras with normal GTPase activity do not act as inhibitors. Gibbs et al., ibid, showed this effect in Zenopus oocytes and in mammalian cells.
Administration of compounds of the invention to block Ras farnesylation not only decreases the amount of Ras in the membrane but also generates a cytosolic pool of Ras. In tumor cells having activated Ras, the cytosolic pool acts as another antagonist of membrane-bound Ras function. In normal cells having normal Ras, the cytosolic pool of Ras does not act as an antagonist. In the absence of complete inhibition of farnesylation, other farnesylated proteins are able to continue with their functions.
Farnesyl-protein transferase activity may be reduced or completely inhibited by adjusting the compound dose. Reduction of farnesyl-protein transferase enzyme activity by adjusting the compound dose would be useful for avoiding possible undesirable side effects resulting from interference with other metabolic processes which utilize the enzyme.
These compounds and their analogs are inhibitors of farnesyl-protein transferase. Farnesyl-protein transferase utilizes farnesyl pyrophosphate to covalently modify the Cys thiol group of the Ras CAAX box with a farnesyl group. Inhibiting HMG-CoA reductase blocks Ras membrane localization in vivo and inhibits Ras function. Inhibition of farnesyl-protein transferase is more specific and is attended by fewer side effects than is the case for a general inhibitor of isoprene biosynthesis.
Previously, it has been demonstrated that tetrapeptides containing cysteine as an amino terminal residue with the CAAX sequence inhibit Ras farnesylation (Schaber et al., ibid; Reiss et. al., ibid; Reiss et al., PNAS, 88: 732-736 (1991)). Such inhibitors may inhibit while serving as alternate substrates for the Ras farnesyltransferase enzyme, or may be purely competitive inhibitors (U.S. Pat. No. 5,141,851, University of Texas).
The compounds of the present invention are peptide analogs containing two reduced peptide bonds and of the general structure C--[.psi.CH.sub.2 NH]Xaa.sup.1 --[.psi.CH.sub.2 NR] Xaa.sup.2 --Xaa.sup.3 where C is Cysteine and Xaa.sup.1-3 is any amino acid and the nitrogen between Xaa.sup.1 and Xaa.sup.2 is alkylated. The compounds of this invention are stable inhibitors of Ras farnesyl-transferase. The presence of the reduced amide linkages confers metabolic stability to these inhibitors such that they are capable of inhibiting ras farnesylation in vivo. Reduction of the first and second peptide bonds may also lead to an unexpected enhancement of intrinsic enzyme-inhibitory activity. Alkylation of the reactive nitrogen between Xaa.sup.1 and Xaa.sup.2 confers chemical stability to these analogs, thus enhancing their activity in vivo (cell culture). Of particular utility is the observation that the lactone or ester forms of these inhibitors are prodrugs that efficiently deliver the active hydroxy acids or acids, respectively, to the intracellular compartment that is the site of Ras farnesylation.
It is, therefore, an object of this invention to develop tetrapeptide-based compounds with two reduced amide linkages, wherein the nitrogen between Xaa.sup.1 and Xaa.sup.2 is alkylated, and which will inhibit farnesyl-protein transferase and the farnesylation of the oncogene protein Ras. It is a further object of this invention to develop chemotherapeutic compositions containing the compounds of this invention, and methods for producing the compounds of this invention.